1. Field of the Invention
The invention concerns the signal broadcasting field for signals including an audio-frequency component. More especially, it concerns a data broadcasting system.
2. Description of the Related Art
The broadcasting field (broadcasting of TV or radio programmes, radiotelephony, etc.) is well known.
A current tendency is to transmit, in addition to the programmes (or sound in the telephony field), data useful for the broadcasting companies, for control organisations, or for listeners or viewers. This data can concern for example:
help in selecting a radio or TV programme (example: automatic tuning aids, search for a radio station by name, search by type of programme, search by menu, etc.), PA1 information on the programme being broadcast or replayed after recording (for example the name of the company which created a programme, the title of a film broadcast by a TV channel, the record reference of a song broadcast by a radio station, etc.), PA1 service data in the analogue radiotelephone field.
We also remark the development of so-called interactive broadcasting systems which allow the viewers or listeners to dialogue in a more or less efficient manner with the programme source. These means are used either to act on the content of the broadcast programme, or to play, bet or communicate on the subject of this same programme. Thus, a form of interactivity, via small devices simulating pseudodialogue with a programme designed for this purpose, recently appeared. A remote-sized unit gives the illusion of interactivity as it allows, for instance, to reply to a televised question/reply game as and when the questions are asked. Or again, an electronic device dissimulated in a fluffy toy allows the toy to react to a broadcast programme or a programme played back on a video cassette recorder. In fact, the interactivity is not real as the string of good replies or the reactions of the toy follows preestablished sequences, common to the memory of the interactive device and the broadcast or played back programme. As the audiovisual sequence was prerecorded in accordance with a selected code, its execution is predictable and therefore the only information to be transmitted to the interactive device is the start signal and the exact timing of the questions/replies or the various possible reactions in the case of a toy.
There is also a demand for the automatic identification of a sound sequence, accompanied by an image or not. For the broadcasters, this is used to check that a given programme is correctly broadcast on the frequency allocated to it; this can become fairly complex when a national programme is affected by regional or local disconnectings. This also allows the controlling bodies to count the broadcasting of works protected by copyrights or to check the conformity of the broadcasting of commercials. Finally, for sample survey or audience evaluation organisations, it is used to rapidly identify that which is actually listened to or seen by a listener or a viewer. Today, to assess a radio audience, the only solution available is to conduct a sample survey by interviewing the consumers.
All these applications are easy to incorporate when designing new radio or TV broadcasting systems, especially digital systems. However, existing systems and equipment populations do not in general easily lend themselves well to this development and experience proves that, from a sales engineering viewpoint, the compatibility and the relative cost of the processes and devices to be implemented are critical factors when introducing a new service.
For the transmission of data concerning a broadcast programme, two techniques are currently used.
The first technique consists in transmitting these data outside of the passband occupied by the signal of the transmitted programme (sound and possibly image). A solution exists, for instance, in sound broadcasting by multiplex frequency modulation, in using the upper part of the multiplex, between 54 and 76 kilohertz. Another example consists in using the lines available during frame retrace for TV broadcasting. These techniques have drawbacks. The saturation of the frequency resources available for broadcasting limits the number of users of these resources. Also, receivers adapted to the passbands used to transmit the emitted information are required.
Another technique consists in transmitting the data in the passband of the signal of the transmitted programme; this technique does not require the use of dedicated frequency bands. It is therefore not necessary to use transmitters and receivers with a frequency adapted to transmit these dedicated frequency bands. Typically, the original signal (corresponding to the programme to be transmitted) is filtered at origin to eliminate the frequency components in a given frequency band and the data is inserted in this band. The original signal is therefore deformed which may be unpleasant for a viewer or a listener not interested in the data. Therefore, the time dedicated to transmitting the information is limited by the broadcasters to the strict minimum which reduces the data flow rate accordingly. Thus, for interactive devices in the television field, the data is loaded globally, in one go, at the start of a given application. It is then impossible to adapt the data subsequent to a modification in the programme which must be run according to scheduled timing and without unexpected interruptions. Filtering means can of course be used at the receivers so as not to systematically pass on the sound or visual data received, this data then being transparent to the listener or the viewer. Nevertheless, we cannot ensure that the signal seen or heard by the viewer or listener will be the same as the original signal that he would have perceived before the insertion of the data.